fix: respect DataFrameWriteOptions::with_single_file_output for paths without extensions

[kumarUjjawal]

Which issue does this PR close?

• Closes Regression: DataFrameWriteOptions::with_single_file_output produces a directory #13323.

Rationale for this change

When using DataFrameWriteOptions::with_single_file_output(true), the setting was being ignored if the output path didn't have a file extension. For example:

df.write_parquet("/path/to/output", 
    DataFrameWriteOptions::new().with_single_file_output(true), 
    None).await?;

Would create a directory /path/to/output/ with files inside instead of a single file at /path/to/output.

This happened because the demuxer used a heuristic based solely on file extension, ignoring the explicit user setting.

What changes are included in this PR?

• Added single_file_output: Option<bool> to FileSinkConfig
• Added test test_single_file_output_without_extension to verify the fix

Are these changes tested?

• New unit test test_single_file_output_without_extension tests the fixed behavior
• All sqllogictest pass

Are there any user-facing changes?

FileSinkConfig now has a new required field single_file_output: Option. Any code that constructs FileSinkConfig
directly will need to add this field. Set to None to preserve existing heuristic behavior.